Multi-layer piezoelectric actuators constituted from piezoelectric layers and internal electrodes stacked alternately one on another have been known as an example of the multi-layer piezoelectric element. The multi-layer piezoelectric actuators can be divided into two categories: fired-at-once type and stacked type that comprises piezoelectric porcelain and internal electrode sheet stacked alternately one on another. When the requirements to reduce the operating voltage and the manufacturing cost are taken into consideration, the multi-layer piezoelectric actuator of fired-at-once type has been recognized to be more advantageous for the reason of smaller layer thickness.
FIG. 6 shows a multi-layer piezoelectric actuator of the prior art. This actuator comprises a stack 53 formed by stacking the piezoelectric layers 51 and the internal electrodes 52 alternately. Placed on both ends of the stack in the direction of stacking are inactive layers 55. The internal electrodes 52 are formed so that one end thereof is exposed at the side face of the stack 53 on the right hand side in one layer and on the left hand side in the next layer, and an external electrode 70 is formed on the side face of the stack 53 where the end of the internal electrode 52 is exposed. The other end of the internal electrode 52 is covered by an insulating material 61 so as to be insulated from the external electrode 70.
The multi-layer piezoelectric actuator of fired-at-once type has been manufactured by stacking a predetermined number of ceramic green sheets formed from a calcined powder of piezoelectric material and an organic binder, with an internal electrode paste made by mixing a silver-palladium powder and a binder being printed on the ceramic green sheets, thereby forming a green compact from the stack, and firing the green compact after removing the organic materials by heating to a predetermined temperature.
The piezoelectric material of the prior art requires a temperature in a range from 1200 to 1300° C. for firing, and accordingly silver-palladium with a high content of expensive palladium has been used to form the internal electrode. With the advancement in the technology of low temperature firing in recent years, piezoelectric materials that can be fired at a temperature around 1100° C. have been developed. With these piezoelectric materials, however, it has been necessary to use silver-palladium consisting of 70% by weight of silver and 30% by weight of palladium when the melting point of the internal electrode is taken into account.
The external electrode has been formed by using an electrically conductive paste consisting of 83 to 99% by weight of silver, 1 to 17% by weight of glass powder and an organic component, applying the paste on the side faces of the stack 53 and firing it at a temperature in a range from 500 to 1000° C. (refer to, for example, Patent Document 1).    Patent Document 1: Japanese Unexamined Patent Publication (Kokai) No. 2000-40635    Patent Document 2: Japanese Unexamined Utility Model Publication (Kokai) No. 60-99522    Patent Document 3: Japanese Unexamined Patent Publication (Kokai) No. 61-133715    Patent Document 4: Japanese Unexamined Utility Model Publication (Kokai) No. 1-130568